1. Field of the Invention
This invention relates to a novel and improved anti-lock control system for motor vehicles, which is operative to prevent the wheels of the motor vehicle from being locked during braking operation of the motor vehicle.
2. Description of the Prior Art
Generally, with an anti-lock system for motor vehicles, anti-lock control is effected by means of microcomputers such that hold valves and decay valves comprising electromagnetic valves are opened and closed on the basis of electrical signals representing wheel speeds sensed by wheel speed sensors, thereby increasing, holding or reducing the brake hydraulic pressure, for the purpose of securing improved steering performance and running stability of the motor vehicle, while at the same time shortening the braking distance.
FIG. 1 of the accompanying drawings illustrate, by way of example, manners in which wheel speed Vw, wheel acceleration and deceleration dVw/dt and brake hydraulic pressure Pw are varied during the operation of the conventional anti-lock control system, together with hold signal HS and decay signal DS for opening and closing hold valves and decay valves, as disclosed in U.S. Pat. No. 4,741,580.
When the brake equipment of the motor vehicle is not being operated while the motor vehicle is running, the hold valves remain open while the decay valves remain closed, the brake hydraulic pressure Pw is not increased; and when the brake equipment is operated, the brake hydraulic pressure Pw is rapidly increased at time t0 so that the wheel speed Vw is decreased (normal mode). A reference wheel speed Vr is set up which is lower by a predetermined amount .DELTA.V than the wheel speed Vw and follows the latter with such a speed difference. More specifically, reference wheel speed Vr is set up so that when the deceleration (negative acceleration) dVw/dt of the wheel reaches a predetermined threshold level, say -1.1 G at time t1, anti-lock control is started, and the reference wheel speed Vr is thereafter made to linearly decrease with a deceleration gradient .theta. (=-1.1 G). At time t2 when the deceleration dVw/dt of the wheel reaches a predetermined maximum value -Gmax, the hold signal HS is generated so that the hold valves are closed, thus holding the brake hydraulic pressure Pw.
With the brake hydraulic pressure Pw being held, the wheel speed Vw is further decreased. At time t3, the wheel speed Vw and the reference wheel speed Vr become equal to each other, and a first cycle of anti-lock control is started; and the decay signal DS is generated, by which the decay valves are opened so that reduction of the brake hydraulic pressure Pw is started. As a result of this reduction of the brake hydraulic pressure Pw, the wheel speed Vw is changed from increase to decrease at time t4 when a low peak VL of the wheel speed Vw occurs. The decay signal DS is interrupted either at the time t4 or at time t5 that is a time point when the wheel speed Vw is increased up to the level of a speed Vb that is higher than the low peak speed VL by 15% of the difference Y between wheel speed Va occurring at the time t3 when the reduction of the brake hydraulic pressure was started and the low peak speed VL, i.e., Vb=VL+0.15Y (FIG. 1 shows the case where the decay signal DS is interrupted at the time t4). Thus, the decay valves are closed so that the reduction of the brake hydraulic pressure Pw is stopped and thus the brake hydraulic pressure is held. The wheel speed Vw is further increased up to the level of a speed Vc that is higher than the low peak speed VL by 80% of the difference Y between the wheel speed Va occurring at the time t3 when the reduction of the brake hydraulic pressure Pw was started and the low peak speed VL, i.e., Vc=VL+0.8Y.
Subsequently, at time t7, a high peak VH of the wheel speed Vw is reached; thereupon the brake hydraulic pressure Pw is again increased. Practically, the high peak VH of the wheel speed Vw is detected in the following manner: The highest one of the four wheel speeds of the vehicle is selected ("select high"); a speed having the acceleration/deceleration of the highest wheel speed limited in the range of .+-.1 G is computed as computed vehicle speed Vv; and when the wheel speed Vw reaches a value Vv -.DELTA.V.sub.o that is lower than the computed vehicle speed by a predetermined amount .DELTA.V.sub.o before a high peak of the wheel speed Vw is actually reached, it is judged that the high peak VH of the wheel speed Vw is reached, whereupon buildup of the brake hydraulic pressure is started. In this case, the buildup of the brake hydraulic pressure Pw is effected in such a manner that the brake hydraulic pressure Pw is alternately increased and held in succession by the fact that the hold signal HS is turned on and off mincingly, or with relatively short intervals so that the brake hydraulic pressure Pw is caused to gradually build up. In this way, the wheel speed Vw is decreased, and at time t8 (corresponding to the time t3, a second cycle of the mode for reduction of the brake hydraulic pressure occurs. An initial brake hydraulic pressure buildup occurs at time t7; the time period Tx of the initial brake hydraulic pressure buildup is determined on the basis of calculation of the average acceleration (Vc -Vb)/.DELTA.V over the time interval .DELTA.T between the time t5 and the time t6 (the average acceleration depends on the friction coefficient .mu. of the road surface); and the time period of the subsequent pressure holding or pressure buildup is determined on the basis of the acceleration or deceleration of th wheel which is detected immediately prior to the pressure holding or pressure buildup. The brake hydraulic pressure increasing, holding and reducing modes are effected in combination as mentioned above; thus, the wheel speed Vw can be controlled so that the vehicle speed can be decreased, while the wheels of the motor vehicle are prevented from being locked.
When the motor vehicle is running on a rough road, it frequently happens that the wheels thereof are caused to float in the air. In such a state, if the brake equipment of the vehicle is being operated, wheels floating in the air will be rapidly decelerated; and when caused to land on the road again, those wheels start rotation again so that changes in the wheel speeds turn out to be different from those which occur when the vehicle is running on a normal road. More specifically, when the vehicle is running on a rough road, the cycle of the anti-lock control turns out to be faster than when the vehicle is running on a normal road, and the amplitude of the wheel speed Vw also becomes greater.
The conventional anti-lock control system is disadvantageous in that in case slow or quick braking is applied when the motor vehicle is running on a rough road, reduction of brake hydraulic pressure will be frequently started due to changes in the wheel speeds so that the brake hydraulic pressure will be prevented from building up, as a result of which the braking distance will be increased.